Why does gender change from singular to plural e.g. El arma; Las armas
A number of feminine nouns that begin with a stressed "a" take the "el" article only because it makes it easier to say. For example "la agua" is hard to pronounce (the two words tend to run together), so we say "el agua", even though "agua" is feminine. All of the other adjectives for these words are feminine: "el agua" but "la fria agua" (yes, I would actually say "el agua fria", but that doesn't demonstrate the point). There is no pronunciation problem with the plurals: "las aguas". "Arma" is an unusual noun since it ends in "ma" but is feminine. Most nouns ending in "ma" or "pa" are masculine.
CalvoViejo, great explanation. He mentioned that most nouns ending in "ma" or "pa" are masculine. Those tend to be the words with Greek roots (such as el mapa and el tema.) Not that you'll know words' roots right off the get-go but it is helpful to learn to recognize Greek-root words as you go.